If for whatever reason you can no longer complete an order you previously accepted just contact Rider Support in the app and ask to be unassigned.
If you’ve been waiting too long at the restaurant to pick up an order, you can unassign yourself in the app. Tap the ‘?’ at the top of the ‘Collect order’ screen. You can see what time the order is estimated to be ready, as well as how long you’ve been waiting so far. Tap ‘Report long wait’ to remind the restaurant you’re there and if you still want to be unassigned, tap ‘Reject order’.
Just remember that you won’t earn fees for any orders you choose to be unassigned from.